cmake: use string(APPEND...) for appending cflags

cmake: use string(APPEND...) for appending cflags

* use string(APPEND...) for less repeatings
* `-rdynamic` is a linker option, so it should be added to
  `CMAKE_EXE_LINKER_FLAGS` for adding symbols to dynamic
  symbol tables so they can be consumed by plugins.

Signed-off-by: Kefu Chai <kchai@redhat.com>